1. Field of the Invention
The present invention relates to thermoplastic polyamide moulding compositions and a process for production of the moulding compositions. The present invention particularly relates to thermoplastic polyamide moulding compositions based on polyamide 12, which are made for further processing by injection moulding or extrusion and which do not result in solid deposits during thermoplastic conversion. The present invention more particularly preferred relates to thermoplastic polyamide 12 moulding compositions based on hydrolytically produced polyamide 12, which do not result in the usual solid deposits during processing in injection moulding or extrusion.
2. Description of Related Art
Polyamide 12 (PA 12) is the type of polyamide which is distinguished by its interesting property profile. Polyamide 12 can be modified in several ways, and the resulting moulding compositions are outstanding thermoplastic in injection moulding and convertible into items of valuable practical use in extrusion processes. Polyamide 12 corresponds to the type of polyamide the properties of which in practical use are the least influenced by changes in temperature and humidity.
One problem is that during the common hydrolytic polymerization process/autoclave treatment the monomer turnover is only 99.5% and the remaining lactam residue is hardly soluble in the polymer, so that, particularly in processing from melts, but also in the later use the lactam 12 (LC 12) is sweat out and sublimed, especially on cooled surfaces, e.g. the surfaces of moulds or rolls and prefabricated parts where it comes to the formation of coatings. Particularly due to the high melting point of lactam 12 those sublimates often form irritating coatings which, particularly when further additives migrate to the surface, give cause for processing troubles with surface damaging and production interruption and so-called “black spots” can also be formed. Well-known measures taken for reduction and elimination of the rest content of lactam 12 are, e.g. the melt- and solid phase postcondensation including vacuum, liquid extraction processes or reprecipitation from alcoholic solution. These processes where the lactam evaporates under thermal influence may as well be disrupted by the lactam sublimate. Furthermore the lactam smoke is highly inflammable and the processes require certain precautionary measures. Furthermore the additional thermal stress can damage the polymer. In thermoplastic processing of polyamide 12 (PA 12) moulding compositions by injection moulding and by extrusion, the formation of solid deposits, particularly those consisting of lactam 12 (LC 12), has a detrimental effect.
U.S. Pat. No. 6,506,830 B1 describes antistatic polyamide moulding compositions with common plasticizers according to the state of the art. The plasticizer concentrations correspond to the usually high plasticizer concentrations (see claim 9; up to 30% by weight). Although U.S. Pat. No. 6,506,830 B1 claim 9 mentions a plasticizer amount of up to 30% by weight and therefore theoretically all concentrations are contained, it is well-known from the literature that small plasticizer additions are regarded as rather giving contrary results due to the “anti-plasticizer-effect” when considering an improvement of material properties. However, in the same context as the plasticizers lactames are mentioned which are the main cause for the formation of deposits.
EP 0 748 847 A2 describes common plasticizer containing polyamide moulding compositions recognisable by the high plasticizer concentrations (preferably 8 to 20% by weight). Furthermore, the advantageous use of plasticizer mixtures is described. The quoted plasticizers also contain sulfonamides and hydroxybenzoates. However, EP 0 748 847 A2 confines itself to the flexibility and cold impact strength of polyamides. Problems of solid deposits are not mentioned. In contrast to this the evaporation of the plasticizer is quoted as a disadvantage (see p. 2, 1. 58 to p. 3, 1. 3). EP 0 748 847 A2 mentions the surface of the mould, but only in the context of a “nontacky surface” which is in no conjunction with the described problems of solid deposits. Apart from that for plasticizers closest attention is paid to vitrification temperature. However, properties at room temperature or higher temperature which are required for the solubility of the polyamide specific solid deposits are not dealt with.
WO 97/10303 uses plasticizers at concentrations of up to 5% by weight for the improvement of welding properties or the increase of joint line strength of the polyamide moulding compositions. Sulfonamides are used as well. But in the same context as the use of plasticizers lactames are mentioned (see p. 4, 1. 35-36). But just the lactames are the cause of the solid deposits.
WO 01/05888 describes moulding compositions which contain, beside polyacetal, flame proving agents and N-containing compounds that can be low molecular. Sulfonamides and urea derivatives belong to these compounds too. The N-containing compounds strengthen the flame protection and increase the stability of the moulding compositions described. In their function as stabilizers concentrations of up to 2.5% are used.
In accordance with WO 01/05888 the moulding compositions must always contain polyacetal and this reference gives no hint to the described problems of solid deposits. This is plausible, because polyamides can be contained in polyacetal, but never play a dominant role there.
EP 0 388 583 A1 is aimed at the fact that the deteriorated cold impact strength of polyamide moulding compositions due to softening can be balanced by certain polyamides. Common plasticizer-containing polyamide moulding compositions are described here with up to 25% by weight plasticizers.
Thus simple, cost saving measures are wanted to avoid the formation of solid deposits in the thermoplastic processing of PA moulding compositions which are based on polyamide 12 or also a copolymer or polyamide respectively, which contains polyamide 12 components as well.